Method and apparatus for frequency offset estimation and correction in orthogonal frequency division multiplexing system

1. A method for estimating and correcting a frequency offset in an orthogonal frequency division multiplexing system, comprising: obtaining a pilot frequency domain channel estimation value of a target user and smoothing the channel estimation value in frequency domain; wherein, the step of obtaining the pilot frequency domain channel estimation value of the target user and smoothing the channel estimation value in frequency domain comprises: obtaining channel estimation values corresponding to two pilots according to a local frequency domain pilot code and a frequency domain receiving sequence of the two pilots received by the target user; and dividing the channel estimation value corresponding to each pilot of the target user into a plurality of sections, and performing frequency domain smoothing in each section; obtaining a link quality state of a receiver: performing frequency offset estimation and frequency offset compensation by a history smoothing algorithm, performing a frequency domain demodulation for the data for which the frequency offset compensation has been performed and outputting one link quality state parameter, according to a history frequency offset value and the pilot frequency domain channel estimation value of the target user when the link quality state satisfies a startup threshold condition, wherein the history frequency offset value is a frequency offset value from a last frequency offset estimation output; performing the frequency offset estimation and frequency offset compensation by a multi-branch attempt algorithm, performing the frequency domain demodulation for the data for which the frequency offset compensation has been performed and selecting to output one link quality state parameter, according to the pilot frequency domain channel estimation value of the target user when the link quality state does not satisfy the startup threshold condition; and updating the link quality state of the receiver according to the link quality state parameter, determining the output value of this frequency offset estimation, and outputting a history frequency offset value required by next frequency offset estimation according to the determined output value of this frequency offset estimation; wherein the history smoothing algorithm comprises: calculating a frequency offset estimation value of the target user by using the history frequency offset value and the pilot frequency domain channel estimation value of the target user, and performing the frequency offset compensation according to the calculated frequency offset estimation value of the target user; and wherein the multi-branch attempt algorithm comprises: calculating the frequency offset estimation value of the target user by using the pilot frequency domain channel estimation value of the target user and each compensation initial value, and performing the frequency offset compensation according to the calculated frequency offset estimation value of the target user.

2. The method according to claim 1 , wherein, the link quality state comprises following parameters: a signal-noise ratio threshold, which acts as a standard of whether the link quality corresponding to the data for which the frequency offset compensation and demodulation has been performed satisfies a requirement; a link quality state register R, wherein a state of the R is 0 or 1, and 1 represents that the link state satisfies the requirement and 0 represents that the link state does not satisfy the requirement; a link quality count threshold, which acts as a startup threshold condition of whether to start the history smoothing algorithm to perform the frequency offset estimation and the frequency offset compensation, wherein, the history smoothing algorithm is stared if greater than the count threshold; and a link quality counter, which is for calculating a number of 1s in the link quality state register, triggering to start the history smoothing algorithm to perform the frequency offset estimation and the frequency offset compensation when a count in the counter is greater than the link quality count threshold; and triggering to start the multi-branch attempt algorithm to perform the frequency offset estimation and the frequency offset compensation when the count in the counter is less than the link quality count threshold.

3. The method according to claim 2 , wherein, the step of performing frequency offset estimation and frequency offset compensation by the history smoothing algorithm, performing a frequency domain demodulation for the data for which the frequency offset compensation has been performed and outputting one link quality state parameter, according to the history frequency offset value and the pilot frequency domain channel estimation value of the target user when the link quality state satisfies the startup threshold condition comprises: performing the frequency offset compensation for an antenna according to the calculated frequency offset estimation value of the target user as per a degree of antenna correlation; demodulating the data for which the frequency offset compensation has been performed in frequency domain and obtaining frequency domain demodulation data; and calculating a signal-noise ratio (SIR) or a check word (CRC) of the demodulated data, wherein the link quality state parameter is the signal-noise ratio or the check word.

4. The method according to claim 3 , wherein, the step of calculating the frequency offset estimation value of the target user by using the history frequency offset value and the pilot frequency domain channel estimation value of the target user comprises: first, calculating this remnant frequency offset ΔΔf ka according to the pilot frequency domain channel estimation values of the target user H 1 (m) (k) and H 2 (m) (k) and the history frequency offset value Δ f n-1 : ΔΔ ⁢ ⁢ f s , ka = ⁢ ∠ ⁡ ( H _ 2 , s , ka ( m ) ) - ∠ ⁡ ( H _ 1 , s , ka ( m ) ) - 2 ⁢ π · L · t · Δ ⁢ ⁢ f _ n - 1 , ka 2 ⁢ π · L · t + l × 2000 = ⁢ ∠ ⁡ ( H _ 2 , s , ka ( m ) ) - ∠ ⁡ ( H _ 1 , s , ka ( m ) ) 2 ⁢ π · L · t - Δ ⁢ ⁢ f _ n - 1 , ka + l × 2000 ΔΔ ⁢ ⁢ f ka = 1 Q ⁢ ∑ s = 1 Q ⁢ ΔΔ ⁢ ⁢ f s , ka , wherein, Δ f n-1,ka is the history frequency offset value from the last frequency offset estimation output, L is a number of orthogonal frequency division multiplexing signals between two pilots, t is a time interval between the orthogonal frequency division multiplexing signals, n represents a sub-frame number, 1 is an integer which makes ΔΔf s,ka ε(−1000,1000), a signal “∠” represents calculating a phase, and Q represents a number of the sections; then, determining that this compensation frequency offset is Δf ka = Δf n-1,ka +ΔΔf ka .

5. The method according to claim 2 , wherein, the step of performing the frequency offset estimation and frequency offset compensation by the multi-branch attempt algorithm, performing the frequency domain demodulation for the data for which the frequency offset compensation has been performed and selecting to output one link quality state parameter, according to the pilot frequency domain channel estimation value of the target user when the link quality state does not satisfy the startup threshold condition comprises: calculating the frequency offset estimation value of the target user according to the pilot frequency domain channel estimation value of the target user and each compensation initial value; performing the frequency offset compensation for an antenna according to the calculated frequency offset estimation value of the target user as per the degree of the antenna correlation; demodulating each of the data for which the frequency offset compensation has been performed in frequency domain and obtaining each of the frequency domain demodulation data; calculating the signal-noise ratio or the check word of each of the demodulated data, wherein the link quality state parameter is the signal-noise ratio or the check word; and selecting a branch of the demodulated data with a greater signal-noise ratio or a correct check word from each of the demodulated data, taking an initial frequency offset value of the branch+the remnant frequency offset value of the branch as an output estimation frequency offset and taking the signal-noise ratio of the branch as an output signal-noise ratio.

6. The method according to claim 5 , wherein, the step of calculating the frequency offset estimation value of the target user according to the pilot frequency domain channel estimation value of the target user and each compensation initial value comprises: first, calculating this remnant frequency offset ΔΔf ka according to the pilot frequency domain channel estimation values of the target user H 1 (m) (k) and H 2 (m) (k) and the initial frequency offset values of different branches f g (m) : ΔΔ ⁢ ⁢ f s , ka = ⁢ ∠ ⁡ ( H _ 2 , s , ka ( m ) ) - ∠ ⁡ ( H _ 1 , s , ka ( m ) ) - 2 ⁢ π · L · t · f g 2 ⁢ π · L · t + l × 2000 = ⁢ ∠ ⁡ ( H _ 2 , s , ka ( m ) ) - ∠ ⁡ ( H _ 1 , s , ka ( m ) ) 2 ⁢ π · L · t - f g + l × 2000 ΔΔ ⁢ ⁢ f ka = 1 Q ⁢ ∑ s = 1 Q ⁢ ΔΔ ⁢ ⁢ f s , ka , wherein, L is a number of orthogonal frequency division multiplexing signals between the two pilots, 1 is an integer which makes ΔΔf s,ka ε(−1000,1000), t is a time interval between the orthogonal frequency division multiplexing signals, a signal “∠” represents calculating a phase, Q represents a number of the sections, g=1, . . . Z, wherein Z is a number of the branches; then, determining that this compensation frequency offset Δf ka,g is Δf ka,g =f g

7. The method according to claim 3 , wherein, the step of updating the link quality state of the receiver according to the link quality state parameter and determining the output value of this frequency offset estimation comprises: the link quality state parameter being the signal-noise ratio, determining that the output of this frequency offset estimation is the history frequency offset value+the remnant frequency offset value, meanwhile updating a latest state position as 1 in the link quality register R and updating the count value of the link quality counter if the signal-noise ratio is greater than the signal-noise ratio threshold; determining the output of this frequency offset estimation is the history frequency offset value, meanwhile updating the latest state position as 0 in the link quality register R if the signal-noise ratio is less than the signal-noise ratio threshold; and determining the history frequency offset value required by the next frequency offset estimation as: a next history frequency offset value=(1−p)*this history frequency offset value+p*this frequency offset estimation, wherein, p is a decimal between 0 and 1.

8. The method according to claim 3 , wherein, the step of performing the frequency offset compensation for the antenna according to the calculated frequency offset estimation value of the target user as per the degree of the antenna correlation comprises: performing the frequency offset compensation for each antenna respectively according to the calculated frequency offset estimation value of the target user when the antenna correlation is weak; performing a unified frequency offset compensation to a plurality of the antennas according to the calculated frequency offset estimation value of the target user when the antenna correlation is strong.

9. The method according to claim 8 , wherein, in the step of the frequency offset compensation, performing the frequency offset compensation for each antenna respectively according to the calculated frequency offset estimation value of the target user when the antenna correlation is weak is to implement the compensation by taking the calculated frequency offset estimation value of the target user as a slope and generating a channel estimation phase of data part by a linear interpolation.

10. An apparatus for estimating and correcting a frequency offset, located in a terminal or a base station to perform frequency offset estimation and compensation for wireless pilot signals, comprising: a pilot frequency domain channel estimation module, configured for obtaining channel estimation values corresponding to two pilots according to a local frequency domain pilot code and a frequency domain receiving sequence of the two pilots received from an antenna by a target user and transmitting the channel estimation values to a smoothing module; the smoothing module, configured for dividing a pilot frequency domain channel estimation value into a plurality of sections and smoothing the pilot frequency domain channel estimation value in each section; a link quality state detection module, configured for performing a link quality detection for the smoothed pilot frequency domain channel estimation value of the target user, obtaining a link quality state and reporting the link quality state to a control module; the control module, configured for notifying a frequency offset estimation and compensation module to perform the frequency offset estimation and frequency offset compensation by a history smoothing algorithm or a multi-branch attempt algorithm according to the link quality state, wherein the history smoothing algorithm comprises: calculating a frequency offset estimation value of the target user by using a history frequency offset value and the pilot frequency domain channel estimation value of the target user, the history frequency offset value being a frequency offset value from a last frequency offset estimation output, and performing the frequency offset compensation according to the calculated frequency offset estimation value of the target user; and wherein the multi-branch attempt algorithm comprises: calculating the frequency offset estimation value of the target user by using the pilot frequency domain channel estimation value of the target user and each compensation initial value, and performing the frequency offset compensation according to the calculated frequency offset estimation value of the target user; the frequency offset estimation and compensation module, configured for, as per an indication of the control module, according to the pilot frequency domain channel estimation value of the target user, performing the frequency offset estimation and the frequency offset compensation according to the pilot frequency domain channel estimation value which has been smoothed by the smoothing module as per the history smoothing algorithm or the multi-branch attempt algorithm, and performing the frequency offset compensation for a plurality of antennas respectively according to a frequency offset estimation result: a data demodulation module, configured for demodulating the data for which the frequency offset compensation has been performed by the history smoothing algorithm or the multi-branch attempt algorithm; and a link quality parameter selection module, configured for performing a link quality parameter selection for the demodulated data for which the frequency offset compensation has been performed by the history smoothing algorithm or the multi-branch attempt algorithm; wherein the control module is further configured for determining this frequency offset estimation output value for which the frequency offset estimation and compensation has been performed by the history smoothing algorithm or the multi-branch attempt algorithm according to a selection result of the link quality parameter selection module, and outputting a history frequency offset value required by next frequency offset estimation according to this determined frequency offset estimation output value.

11. The apparatus according to claim 10 , wherein, the control module is configured for notifying the frequency offset estimation and compensation module to perform the frequency offset estimation and compensation according to the history frequency offset value and the pilot frequency domain channel estimation value of the target user when the link quality state satisfies a startup threshold condition; and the frequency offset estimation and compensation module is further configured for performing the frequency offset estimation and the frequency offset compensation by the history smoothing algorithm, performing a frequency domain demodulation for the data for which the frequency offset compensation has been performed and outputting one link quality state parameter; or the control module is configured for notifying the frequency offset estimation and compensation module to perform the frequency offset estimation and compensation according to the pilot frequency domain channel estimation value of the target user when the link quality state does not satisfy the startup threshold condition; and the frequency offset estimation and compensation module is further configured for performing the frequency offset estimation and the frequency offset compensation by the multi-branch attempt algorithm, performing the frequency domain demodulation for the data for which the frequency offset compensation has been performed and outputting one link quality state parameter.

12. The apparatus according to claim 10 , wherein, the link quality state detection module comprises a link quality state register and a link quality state counter, the link quality state register is configured for storing the link quality state; and the link quality state counter is configured for accumulating a number of the link quality states which satisfy the condition.

13. The apparatus according to claim 12 , wherein, the control module is further configured for notifying the link quality state module to update the state in the link quality register and digits in the link quality state counter after determining this final frequency offset estimation output value.

14. The method according to claim 5 , wherein, the step of updating the link quality state of the receiver according to the link quality state parameter and determining the output value of this frequency offset estimation comprises: the link quality state parameter being the signal-noise ratio, determining that the output of this frequency offset estimation is the history frequency offset value+the remnant frequency offset value, meanwhile updating a latest state position as 1 in the link quality register R and updating the count value of the link quality counter if the signal-noise ratio is greater than the signal-noise ratio threshold; determining the output of this frequency offset estimation is the history frequency offset value, meanwhile updating the latest state position as 0 in the link quality register R if the signal-noise ratio is less than the signal-noise ratio threshold; and determining the history frequency offset value required by the next frequency offset estimation as: a next history frequency offset value=(1−p)*this history frequency offset value+p*this frequency offset estimation, wherein, p is a decimal between 0 and 1.

15. The method according to claim 5 , wherein, the step of performing the frequency offset compensation for the antenna according to the calculated frequency offset estimation value of the target user as per the degree of the antenna correlation comprises: performing the frequency offset compensation for each antenna respectively according to the calculated frequency offset estimation value of the target user when the antenna correlation is weak; performing a unified frequency offset compensation to a plurality of the antennas according to the calculated frequency offset estimation value of the target user when the antenna correlation is strong.